1. Field of the Invention
The present invention relates to a method for cutting a rare earth alloy, a method for manufacturing a rare earth magnet, and a wire-saw machine. More particularly, the present invention relates to a method for cutting a rare earth alloy using a sawing-wire with abrasive grains fixed to a core wire, and a method for manufacturing a rare earth magnet and a wire-saw machine adopting this cutting method.
2. Description of Related Art
Rare earth alloys have been used as strong magnet materials, for example. Rare earth magnets, obtained by magnetizing rare earth alloys, are suitably used as magnets for voice coil motors used for positioning a magnetic head of a magnetic recording apparatus, for example.
Conventionally, for cutting ingots (including sintered bodies) of rare earth alloys, a technique of slicing an ingot with a rotary slicing blade, for example, has been adopted. This technique using a slicing blade, however, requires an undesirably large cutting margin because the cutting edge of the slicing blade is comparatively thick. This reduces the yield of the rare earth alloy materials and therefore raises the cost of rare earth alloy products (rare earth magnets, for example).
A cutting method using a sawing wire is known as a method requiring a smaller cutting margin than the method using a slicing blade. For example, Japanese Laid-Open Patent Publication No. 11-198020 discloses that hard and brittle materials such as silicon, glass, neodymium, and ferrite can be cut using a sawing wire with abrasive super-fine grains fixed to the circumference of a high-strength core wire via a bond layer (hereinafter, this type of wire is called an “abrasive grain-fixed wire”).
The manufacture cost of rare earth magnets will be widely reduced if an ingot of a rare earth alloy can be cut using an abrasive grain-fixed wire as described above to produce a large number of plates having a predetermined thickness simultaneously with a reduced cutting margin. However, no report has ever been made on success of cutting of a rare earth alloy using an abrasive grain-fixed wire in a mass-production scale.
The present inventors have examined the above matter in various aspects and found that a main reason for the failure of cutting of a rare earth alloy using an abrasive grain-fixed wire in a mass-production scale is that the mechanical properties of a rare earth alloy, in particular, a rare earth alloy produced by a sintering method (hereinafter, referred to as a “rare earth sintered alloy”) are greatly different from those of silicon and the like. Specifically, a rare earth sintered alloy, which is rather brittle as a whole and has a main phase (i.e., R2Fe14B crystal grain) having a relatively high hardness and a grain boundary phase causing ductile fracture, is hard to be cut, unlike the hard and brittle material such as silicon. In other words, a rare earth sintered alloy exhibits high cutting resistance and as a result generates a large amount of heat, compared with the material such as silicon. In addition, the specific gravity of a rare earth alloy is about 7.5, which is large compared with that of silicon and the like, indicating that saw dust (sludge) generated by the cutting are settled and not easily discharged from a cut portion.
In view of the above, in order to cut a rare earth alloy efficiently with high machining precision, it is necessary to sufficiently reduce the cutting resistance and also efficiently release heat generated during the cutting, that is to say, efficiently cool a cut portion. It is also necessary to efficiently discharge saw dust generated by the cutting.
The cutting resistance can be reduced and heat generated during the cutting can be dissipated efficiently, by supplying a coolant (also called a “cutting fluid”) excellent in lubricity to a cut portion of a rare earth alloy. According to the results of experiments conducted by the present inventors, by wetting a sawing wire with a sufficient amount of an oil-base coolant, the coolant can be sufficiently delivered to a narrow cut portion by the traveling wire (see U.S. patent application Ser. No. 09/662,136, for example).
However, use of an oil-base coolant has the following problems. It costs high to dispose of waste of the oil-base coolant to ensure not to cause environmental disruption. Also, it is difficult to separate saw dust (i.e., magnetic particles) from the waste and thus difficult to reuse the waste and the saw dust. In consideration of these, it appears suitable to use water (or a water-soluble liquid) as the coolant. However, since water is low in viscosity (1.0 mm2/s), a sufficient amount of water is not allowed to adhere to a traveling wire, and therefore it is not possible to deliver a sufficient amount of water to a cut portion even when the wire is wet with water.
Japanese Laid-Open Patent Publication No. 11-198020 discloses that a coolant is allowed to adhere to an abrasive grain-fixed wire without fail even when the wire is traveling at high speed (for example, 2000 m/min) by making the wire travel in the coolant overflowing a coolant reservoir. However, according to experiments conducted by the present inventors, when a rare earth alloy is cut with a sawing wire that is traveling in overflowing water (as disclosed in Japanese Laid-Open Patent Publication No. 11-198020, for example), there arise problems that abrasive grains drop off and, in an extreme case, the wire snaps. These problems occurred even when the wire travel speed was as low as about 800 m/min, for example. This is probably because a sufficient amount of water was not delivered to a cut portion even by adopting the above method.
From another examination by the present inventors, it has been found that when a coolant containing water as the main component is used, abrasive grains tend to drop off the wire due to contact friction between adjacent windings of the wire on a reel bobbin around which the wire is wound (this phenomenon is sometimes called shedding or detachment).
The reason is found as follows. The coolant containing water as the main component is low in adhesion to the wire so as to be easily shaken off and also easily evaporates, compared with an oil-base coolant. Therefore, only a small amount of coolant, or virtually no coolant, is kept adhering to the wire when the wire is wound around the reel bobbin. With lack of a sufficient amount of coolant, it is unable to reduce heat generation, as well as mechanical friction force, due to friction between adjacent wire windings. In other words, it is presumed that although the coolant is supplied to the wire at the cut portion, the coolant on the wire is flung off during the travel of the wire before the wire is wound around the reel bobbin.
The inter-wire friction mechanically damages abrasive grains, even though it does not cause shedding of abrasive grains, resulting in reducing cutting precision and cutting efficiency. In a worse case, the bond layer may be peeled off together with the abrasive grains fixed thereto. In other words, when a coolant containing water as the main component is used, the life of the wire is shortened due to the inter-wire friction on the reel bobbin. Since an abrasive grain-fixed wire is comparatively expensive, it is desirable to make the life of the wire longer for at least reduction of the cost for the cutting.
It has also been found that the wire snaps with high frequency when a coolant containing water as the main component is used, compared with the case of using an oil-base coolant. This also shortens the life of the wire.